As a means for preventing global warming attributed to CO2 emission, hydrogen gas fuels are attracting attention. today as alternatives for oil and coal fuels (fossil fuels) that have depended so far in relation with environmental technology, and development of various systems such as power generation, cooling, and storage using hydrogen gas as fuels has been energetically carried out. In due course of developing such systems, an expansion in the applications of rare earth metal-based permanent magnets, such as R—Fe—B based permanent magnets represented by a Nd—Fe—B based permanent magnet, is expected because the magnets are made from low cost materials that are abundant in resources and have superior magnetic characteristics, and if embedded in circulation motors and magnetic sensors that are used in supplying or transporting hydrogen gases, they can realize low cost compact systems.
In the case of considering extending the application of rare earth metal-based permanent magnets in the fields using hydrogen gas as fuels, the magnets should have hydrogen resistance during usage which resist to environments of high hydrogen gas pressure. However, considering a case of an R—Fe—B based permanent magnet, for instance, the magnet possesses high hydrogen absorptivity as is clear from the fact that this magnet is manufactured through the process of finely dividing magnetic powder by pressurized crushing using hydrogen gas. Accordingly, in case hydrogen gas is present in the environment the magnet is used, there may be assumed an environment in which the hydrogen gas pressure may be 100 kPa or higher regardless of whether the environment is made up of hydrogen gas alone or formed of a mixed gas of hydrogen gas and other gases; in such a case, there is such a problem that, if sufficient hydrogen resistance should not be imparted to the magnet, the magnet may absorb hydrogen and become brittle due to the reaction of R with hydrogen, thereby causing the formation of hydrides or an exothermic reaction, finally ending in the disruption of the magnet.
As a method for imparting hydrogen resistance to rare earth metal-based permanent magnets, there is proposed, for instance, a method according to patent reference 1 below, which comprises forming a Cu coating film on the surface of the magnet, and further forming on the surface thereof a metal coating film made of a metal more base than Cu, such as a Ni coating film. However, this method is merely a method for preventing the magnet from absorbing hydrogen gas that is generated during the formation of the metal coating film on the surface of the magnet. Accordingly, simple adoption of such a layered structure is not sufficient for imparting hydrogen resistance to the magnet for use under high hydrogen gas pressure environment such as 100 kPa or higher; in particular, under such an environment that hydrogen is diffused and evolved repeatedly to the metal coating film, there is a problem as such that the generation of blistering and peeling off of the coating film is provoked, thereby leading to an abrupt rupture of the magnet and the metal coating film at an early stage, and a disruption of the magnet. Further according to patent reference 2 below, there is proposed a method which comprises forming, on the surface of the magnet, a multilayered metal coating film having 4 layers or more of Ni coating film(s) and Cu coating film(s) as the constituent coating films, and the total film thickness is in a range of from 15 μm to 70 μm, provided that the thickness of the Cu coating film(s) accounts for 30% or more of the total film thickness. However, this method suffers problems such as a reduction in effective volume of the magnet and an increase in cost.
Patent reference 1: JP-A-5-29119
Patent reference 2: JP-A-2003-166080